This summer alone, intense heat waves have blamed at least 11 deaths in Japan, a record-breaking 45.9-degree Celsius temperature in France, and hot advice hitting 147 million people on the U.S. east coast. Guessed as the "new normal", these extreme temperatures in the air can heat our bodies, causing sunrays or even organ damage.
A new study by researchers at the Lawrence Berkeley National Laboratory of the Department of Energy (Berkeley Lab) shows that if every building in California sported "cold" roofs by 2050, these roofs would help to help protect cities from the consequences of these dangers heat waves. Their study, "Interactive implications of climate change, population dynamics, and urban warming for future exposure to hot extremes," was recently published in the journal. Environmental Research Letters.
The researchers predict that heat waves are likely to become two to 10 times more common across the state by the middle of the century. But if cool roofs were adopted across California's most populous regions – the San Francisco Bay, Los Angeles, San Diego, and Sacramento – by 2050 these reflective roofs could lower warm waves (defined as each time a person experiences a heat wave). 35 million annually, compared to an estimated 80 million hot cases in 2050 with no increase in cool-roof adoption. This is the latest example from Berkeley Lab's research into the potential of reflective roofs, walls and pavements to mitigate urban heat, reduce air conditioning use and save water.
"Urban spaces are a small fraction of the globe, but they are where most people live," said Pouya Vahmani, a postdoctoral research fellow in the Berkeley Lab's Division of Climate and Ecosystem Sciences and lead author of the study. "If we are able to cool those areas even a little bit, it can have a huge impact on health and get major impacts from climate change."
Predicting future heat waves
Urban residents are more vulnerable than their rural counterparts to the negative effects of heat. Due to the urban heat island effect, air temperatures peak a few degrees higher in cities than in surrounding vegetation regions. Built-in surfaces easily absorb daytime heat and release it at night, keeping cities warmer at night.
When night temperatures remain high, human bodies struggle to cool off and recover from the scorching heat of the day. In summer, when urban hot islands are effective, human health can be seriously compromised. Add heat to this already heated environment, and the effects worsen.
In this new study, Berkeley Lab researchers had two goals. First, they wanted to predict heat waves across California's 29 major urban counties between now and 2050. They used regional climate conditions between 2001 and 2015 as a starting point to simulate mid-century climate under two hot scenarios.
Combining these climate conditions with high-resolution satellite imagery has allowed them to incorporate urban features such as buildings, roads and vegetation, which absorb and release heat and for more accurate future climate forecasts. Subsequently, the researchers used county-level estimates for 2050 to estimate population to future heat waves.
"We wanted to get a better picture of future climate change for California's urban environments and adaptation choices," said Andrew Jones, a scientist at the Berkeley Lab's Department of Climate and Ecosystem Sciences and co-author of the study. "Making such refined and realistic forecasts can help urban planners and citizens prepare for hot events in an increasingly hot future."
Their study found that heat waves with air temperatures exceeding 35 degrees Celsius (95 degrees Fahrenheit) and lasting at least three consecutive days become two to 10 times more common in future global warming scenarios. With the additional burden of urban centers increasing, researchers expect that by 2050 there will be 80 million cases of heat stroke in California annually, compared to an average of 37 million cases annually under current climate conditions.
"That's more than double the cases we've seen under current climate conditions," Vahmani said.
Also regions such as downtown Los Angeles and Santa Clara, Alameda and Orange Prefectures, which have historically benefited from cold sea winds, will experience more frequent, more intense heat waves by the middle of the century. "This is important," Vahmani said, "because many of these regions are not ready to deal with the extreme heat."
Cool roofs to the rescue
Fixing white roofs or installing sunroof reflections in urban areas could be one solution, according to the researchers.
"It's not necessarily a complicated or expensive technology," Vahmani said. "But cold roofs are generally more effective if everyone adopts them."
The second aim of their study was to analyze the efficiency of cold roofs in mitigating heat wave effects. To this end, the research team repeated the same high-resolution regional climate simulations, only this time replacing all existing building roofs with cold roofs. Specifically, they wanted to know if the increased solar reflectance might shield urban populations against heat waves.
They found that if every building in California sported cool roofs by 2050, it could reduce the annual number of heat waves in California to 45 million from 80 million.
This powerful attenuation surprised the team. "Although a small percentage of California's land is urban, I was surprised at how effective cold roofs could be in pushing back the risks of extreme heat," Jones said.
In addition to reduced exposure to heat waves and related health risks, this study predicts that widespread adoption of cold roofs could dramatically reduce energy demand by reducing climate use.
Jones acknowledges that 100% conversion to cool roofs by 2050 may be difficult. While cities like Los Angeles are ordering cool roofs for new construction, restoring existing buildings can be expensive.
The team now wants to find the minimum cooling roof coverage needed to accumulate benefits similar to those reported in the study.
But the positive effect of cold roofs will be limited to reducing daytime temperatures when the roofs reflect sunlight. At night, when roads and crowded buildings slowly release heat, these roofs cannot directly provide a cooling advantage.
"What they can do is reduce the amount of hot cities absorbing during the day," Jones said, "which can indirectly lower the heat available for release at night."
Reflective roofs can reduce heating in cities and save lives during hot waves
P Vahmani et al, Interactive implications of climate change, population dynamics and urban warming for future exposure to hot extremes, Environmental Research Letters (2019). DOI: 10.1088 / 1748-9326 / ab28b0
Cold roofs can help shield the cities of California against heat waves (2019, August 14)
Retrieved August 14, 2019
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